The present invention relates to a series motor, in particular universal motor, with a brake device.
Series motors which are frequently identified as universal motors are utilized for example as electrical hand tools such as shears, chain saws, lawn mowers, grinding tools, circular saws or electrical planes. After turning off the system voltage, these machines, due to the kinetic energy of their drives, have a long post-running time. The post-running cutters, shears, saws, grinding discs or chains pause for the operators of these devices a substantial injury risk.
It is therefore advantageous, during turning off of the system voltage to provide a fast braking of the motor and thereby the tool.
It is known to use mechanical braking devices for braking the motor. They however are less suitable since they have the construction which is complicated, expensive and wear prone.
It is further known to provide an electric braking of universal motors by a self-excited short circuiting on the permanent induction in the magnetic circuit of the motor. This however has this disadvantage that the voltage induced by the permanent induction in the armature is not always sufficient to reliably initiate the self-excitement of the universal motor. For removing these disadvantages it has been proposed to perform the ignition of the braking process by a capacitor which is charged through a diode during the operation time. This is however disadvantageous since the ignition process can be performed only once, so that when it is elapsed a new ignition is no longer possible.
The German patent document DE-OS 38 37 943 discloses an approach in accordance with which the ignition is performed by a capacitor which is loaded after turning off of the motor with alternative voltage. Here, the current flowing through the capacitor is produces in the field coil located outside the braking circuit an electric field which induces in the rotatable armature a voltage sufficient for reliable ignition of the braking process.
In addition to a substantially great additional mounting space for the capacitor, this approach has the disadvantage that the capacitor and thereby a field winding of the plugged-in device the total time depends on the electrical system. Since for reliable ignition of the braking process correspondingly greatly dimensioned capacitors are required, the current which flows through the capacitor causes an imbalanced interference.